In the oil patch, power tongs are used to turn and make-up joints on tubing or drill rod as small as 11/2 inches in diameter, and on pipe or casing ranging up to 16 inches in diameter, or more.
As an example of a power tong of the type which may employ the invention herein, reference may be made to U.S. Pat. No. 4,350,062 Farr et al. This patent describes a power tong having a power-driven rotary gear which has a camming surface formed along its inner circular face. This gear carries a jaw assembly as it rotates, the jaws being contained between cage plates that cap the upper and lower sides of the rotary gear. Both the rotary gear and the cage plates are "C"-shaped to provide access openings by which drill pipe may be placed centrally within these components.
The cage plates are free for rotation with respect to the rotary gear about their common center. However, the cage plates carry jaw assemblies that extend between the camming surface on the inside of the rotary gear, and the pipe which is to be turned. When the cage plate is rotated with respect to the rotary gear, cam followers on the jaw assemblies are advanced radially inward by the camming surface, until the jaws engage with the drill pipe. Further advancement of the cam followers up the cam surface locks the jaws to the pipe and arrests further relative rotation between the cage plates and the rotary gear. Thereafter, the pipe is turned by the continued rotation of the rotary gear, the force to effect rotation being transmitted through the jaws which are engaged with the pipe.
When the jaws are not in use they may be withdrawn from the central portion of the tong by the provision of a neutral recess formed in the rotary gear. This recess is located adjacent to the camming surface so that the cam follower roller may retire into it, allowing the jaws to swing outwardly from the tong centre.
The proper grasping of the drill pipe by the jaws depends on the relative rate of advancement of the jaws inwardly as the cam follower moves along the cam surface. The cam surface may be envisaged as a kind of curved wedge that is forced against the cam follower to urge the jaws inwardly. As with a wedge, the rate of increase of the inward gripping force applied by the jaws as the cam follower moves up the cam surface will depend upon the steepness of the cam surface. This ratio may be characterized in the "camming schedule".
If it is desired to convert a larger-size power tong, for example a 133/8 inch tong, for use on smaller pipe, it may first be thought that the jaws need merely be made smaller and displaced towards the center, as by the insertion of spacers. However, the cam schedule necessary for use on larger size pipe will not reliably produce the gripping force needed on smaller pipe. Instead, such a straight forward modification will result in jaws which slip and score the smaller drill pipe.
A modification that will permit a larger power tong to be used on small pipe without causing slippage or scoring is therefore desirable.
Two patents which address this issue are U.S. Pat. No. 4,250,773 to Haynes, and U.S. Pat. No. 5,044,232 to Schulze-Beckinghausen.
In the Haynes patent, interchangeable jaws are described in which substitute cam follower assemblies for smaller pipe engage the same cam surfaces as used for larger pipe (column 3 line 68). In the Schulze-Beckinghausen reference, rollers of different diameter, and optionally different jaws, are substituted when it is desired to rotate pipe of a substantially different size. Unlike the more typical tong arrangement described above, the jaws of this design carry the cam surface and the cam following roller is mounted on the rotary gear. No reference is made, however, to the presence of a different camming surface, having a different camming schedule, in accordance with the size of pipe to be engaged.
Weatherford U.S. Inc., the assignee of the Schulze-Beckinghausen patent has published a brochure that refers to the use of an auxiliary cam arrangement, one separate auxiliary cam being used for jaws accommodating 4 to 51/2 inch casing; and a second auxiliary cam being used for 23/8 to 31/2 inch casing. No reference is made, however, in this brochure as to the manner by which these auxiliary cams are employed within the tong.
It is with this background in mind that this invention has as an object to provide an improved means by which power tongs may be adapted for use with rod, tubing, pipe or casing having differing diameters.
The invention herein in its general form, will next be summarized, and then its implementation in the form of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. As such they are merely exemplary. The invention will then be further described, and defined, in its most general and more specific forms by means of the series of claims which conclude this Specification.